790 Comparative Morphology of Chordates 



order. Comparative Anatomy, using only its own data, asserts the 

 basic structural similarity of all vertebrates, but it cannot confidently 

 assert more than that. Its data suffice merely to suggest a theory of the 

 genetic continuity of animals, or "evolution." But, left to itself, 

 Anatomy must have continued to use such terms as "basic structure," 

 "plan," or "pattern," variously (but by what means?) adapted. These 

 terms belong to the language of transcendentalism. Comparative 

 Embryology discovers a common early-embryonic basis for all verte- 

 brates. But, in the later embryonic stages, there is an extraordinary 

 degree of indirectness in the manner of development of many of the 

 organs. Primary embryonic structures may be torn down and rebuilt, 

 as when cartilaginous skeletal parts are replaced by bone. A notochord 

 is replaced by cartilaginous vertebrae, which are later replaced by bone. 

 Pronephros and mesonephros may be more or less completely obliter- 

 ated and replaced by a metanephros. A simple median postcaval vein 

 emerges from a prolonged and intricate process of rebuilding and trans- 

 forming the primary venous drainage system of the embryonic trunk. 

 In many instances it seems as if the embryo takes a long, devious, and 

 unnecessarily laborious way of arriving at a simple definitive result. 

 Especially significant is the fishlike structure and mode of life of the 

 amphibian larva, and the transitory presence of reptilian character- 

 istics during the embryonic development of mammals and birds — 

 especially conspicuous in the development of the skull and other 

 skeletal parts. 



These striking peculiarities in the embryonic development of 

 vertebrates seem to offer only one reasonable interpretation — the 

 genetic continuity of a vertebrate series ranging from fishes through 

 amphibians and then reptiles to a duplex climax in mammals and birds. 

 If any doubt as to the correctness of this interpretation could remain, 

 it should be dispelled by the chronology of the vertebrate series as 

 revealed by Paleontology and by the fact that Paleontology provides 

 many transitional types or "connecting links" which do much to fill 

 the gaps between the groups of existing vertebrates. The basic struc- 

 ture of the ancestral fishlike vertebrate persists in the early embryo 

 throughout the series. In the course of the embryonic development of 

 later members of the series appear transitory characteristics inherited 

 from successive ancestors — as, e.g., in the development of the aortic 

 vessels of a bird, several aortic arches arranged as in fishes are reduced 

 to a temporary arrangement similar to that of reptiles (see p. 625), to 

 hv followed by further reduction leaving onl> the right member of the 

 fourth arch in the adult bird. According to Ernst Haeckel (see p. 349), 

 "ontogeny repeats phylogeny " — and it surely does, at least in develop- 

 ment of the basic vertebrate structures. 



